Pixel measurement through data line

ABSTRACT

A system and method for determining the current of a pixel circuit and an organic light emitting diode (OLED). The pixel circuit is connected to a source driver by a data line. The voltage (or current) supplied to the pixel circuit by the source driver. The current of the pixel and the OLED can be measured by a readout circuit. A value of a voltage from the measured current can be extracted and provided to a processor for further processing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/028,073, filed Jul. 5, 2018, now allowed, which is acontinuation-in-part of U.S. patent application Ser. No. 15/968,134,filed May 1, 2018, which claims the benefit of U.S. ProvisionalApplication No. 62/629,450, each of which is hereby incorporated byreference herein in their entireties.

BACKGROUND

Organic light emitting diode (OLED) displays have gained significantinterest recently in display applications in view of their fasterresponse times, larger viewing angles, higher contrast, lighter weight,lower power, amenability to flexible substrates, as compared to liquidcrystal displays (LCDs).

OLED displays can be created from an array of light emitting deviceseach controlled by individual circuits (i.e., pixel circuits) havingtransistors for selectively controlling the circuits to be programmedwith display information and to emit light according to the displayinformation. Thin film transistors (“TFTs”) fabricated on a substratecan be incorporated into such displays. TFTs tend to demonstratenon-uniform behavior across display panels and over time as the displaysage. Compensation techniques can be applied to such displays to achieveimage uniformity across the displays and to account for degradation inthe displays as the displays age. Some schemes for providingcompensation to displays to account for variations across the displaypanel and over time utilize monitoring systems to measure time dependentparameters associated with the aging (i.e., degradation) of the pixelcircuits. The measured information can then be used to inform subsequentprogramming of the pixel circuits so as to ensure that any measureddegradation is accounted for by adjustments made to the programming. Theprior art monitored pixel circuits, however, require the use ofadditional feedback lines and transistors to selectively couple thepixel circuits to the monitoring systems and provide for reading outinformation. The incorporation of additional feedback lines andtransistors may undesirably add significantly to the cost yield andreduces the allowable pixel density on the panel.

SUMMARY OF THE INVENTION

Aspects of the present disclosure include a method of determining thecurrent of a pixel circuit connected to a source driver by a data line.The method includes supplying voltage (or current) to the pixel circuitfrom the source via the data line, measuring the current and extractingthe value of the voltage from the current measurement. The pixel circuitmay include a light-emitting device, such as an organic light emittingdiode (OLED), and may also include a thin field transistor (TFT).

In this aspect of the present disclosure further includes the sourcedriver having a readout circuit that is utilized for measuring thecurrent provided by the source driver to the pixel circuit. The currentis converted into a digital code, i.e. a 10 to 16 bit digital code. Thedigital code is provided to a digital processor for further processing.

The foregoing and additional aspects and embodiments of the presentinvention will be apparent to those of ordinary skill in the art in viewof the detailed description of various embodiments and/or aspects, whichis made with reference to the drawings, a brief description of which isprovided next.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an OLED display in accordance withembodiments of the present invention.

FIG. 2 is a block diagram of an embodiment of a pixel driver circuit inprogramming mode for the OLED display in FIG. 1.

FIG. 3 is a block diagram of an embodiment of a pixel driver circuit inmeasurement mode for the OLED display in FIG. 1.

FIG. 4 is a block diagram of an embodiment of a pixel driver circuit innormal operation mode for the OLED display in FIG. 1.

FIG. 5 is a block diagram of an embodiment of a pixel driver circuit inprogramming mode which is not selected by the Enable Management signalfor the OLED display in FIG. 1.

FIG. 6 is a block diagram of an OLED display in accordance withembodiments of the present invention.

FIG. 7 is a block diagram of an embodiment of a pixel circuit whichincludes two TFTs, T1 and T2, an OLED and a capacitor.

FIG. 8 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) in programming mode.

FIG. 9 is a block diagram of an embodiment of a column of pixel circuit(“jth” column). In this mode, data line has the same voltage as supplyvoltage (VDD) and all capacitors' voltages are set to be zero and OLEDdevices show black color.

FIG. 10 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) in measurement mode. The leakage current is measured inthis mode.

FIG. 11 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) in programming mode. In this mode the “ith” row isprogrammed.

FIG. 12 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) in measurement mode. The pixel current of the “ith” pixelplus the leakage currents of the other pixels are measured in this mode.

FIG. 13 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) in measurement mode. The OLED current of the “ith” pixelplus the leakage currents of the other pixels are measured in this mode.

DETAILED DESCRIPTION

FIG. 1 is a diagram of an exemplary display system 10. The displaysystem 10 includes a gate driver 12, a source driver 14, a digitalcontroller 16, a memory storage 18, and display panel 20. The displaypanel 20 includes an array of pixels 22 arranged in rows and columns.Each of the pixels 22 is individually programmable to emit light withindividually programmable luminance values. The controller 16 receivesdigital data indicative of information to be displayed on the displaypanel 20. The controller 16 sends signals 32 to the source driver 14 andscheduling signals 34 to the gate driver 12 to drive the pixels 22 inthe display panel 20 to display the information indicated. The pluralityof pixels 22 associated with the display panel 20 thus comprise adisplay array (“display screen”) adapted to dynamically displayinformation according to the input digital data received by thecontroller 16. The display screen can display, for example, videoinformation from a stream of video data received by the controller 16.The supply voltage 24 can provide a constant power voltage or can be anadjustable voltage supply that is controlled by signals from thecontroller 116. The display system 10 can also incorporate features froma current source or sink (not shown) to provide biasing currents to thepixels 22 in the display panel 20 to thereby decrease programming timefor the pixels 22.

For illustrative purposes, the display system 10 in FIG. 1 isillustrated with only four pixels 22 in the display panel 20. It isunderstood that the display system 10 can be implemented with a displayscreen that includes an array of similar pixels, such as the pixels 22,and that the display screen is not limited to a particular number ofrows and columns of pixels. For example, the display system 10 can beimplemented with a display screen with a number of rows and columns ofpixels commonly available in displays for mobile devices, monitor-baseddevices, and/or projection-devices.

The pixel 22 is operated by a driving circuit (“pixel circuit”) thatgenerally includes a driving transistor and a light emitting device.Hereinafter the pixel 22 may refer to the pixel circuit. The lightemitting device can optionally be an organic light emitting diode, butimplementations of the present disclosure apply to pixel circuits havingother electroluminescence devices, including current-driven lightemitting devices. The driving transistor in the pixel 22 can optionallybe an n-type or p-type amorphous silicon thin-film transistor, butimplementations of the present disclosure are not limited to pixelcircuits having a particular polarity of transistor or only to pixelcircuits having thin-film transistors. The pixel circuit 22 can alsoinclude a storage capacitor for storing programming information andallowing the pixel circuit 22 to drive the light emitting device afterbeing addressed. Thus, the display panel 20 can be an active matrixdisplay array.

As illustrated in FIG. 1, the pixel 22 illustrated as the top-left pixelin the display panel 20 is coupled to a power enable (PE) signal line40, measurement (MEAS) signal line 42, a supply line 26 i, a data line23 j, and an enable measurement (EM) signal line 44 i. The supply line26 i may be charged with VDD.

The top-left pixel 22 in the display panel 20 can correspond a pixel inthe display panel in a “ith” row and “jth” column of the display panel20. Similarly, the top-right pixel 22 in the display panel 20 representsa “jth” row and “mth” column; the bottom-left pixel 22 represents an“nth” row and “jth” column; and the bottom-right pixel 22 represents an“nth” row and “mth” column. Each of the pixels 22 is coupled to the PEsignal line 40, MEAS signal line 42; along with the appropriate supplylines (e.g., the supply lines 26 i and 26 n), data lines (e.g., the datalines 23 j and 23 m), and EM signal lines (e.g., the EM signal lines 44i and 44 n). It is noted that aspects of the present disclosure apply topixels having additional connections, such as connections to a selectline.

With reference to the top-left pixel 22 shown in the display panel 20,PE signal line 40 and MEAS signal line 42 are provided by the gatedriver 12, and can be utilized to enable, for example, a programmingoperation of the pixel 22 by activating a switch or transistor to allowthe data line 23 j to program the pixel 22. The data line 23 j conveysprogramming information from the source driver 14 to the pixel 22. Forexample, the data line 23 j can be utilized to apply a programmingvoltage or a programming current to the pixel 22 in order to program thepixel 22 to emit a desired amount of luminance. The programming voltage(or programming current) supplied by the source driver 14 via the dataline 23 j is a voltage (or current) appropriate to cause the pixel 22 toemit light with a desired amount of luminance according to the digitaldata received by the controller 16. The programming voltage (orprogramming current) can be applied to the pixel 22 during a programmingoperation of the pixel 22 so as to charge a storage device within thepixel 22, such as a storage capacitor, thereby enabling the pixel 22 toemit light with the desired amount of luminance during an emissionoperation following the programming operation. For example, the storagedevice in the pixel 22 can be charged during a programming operation toapply a voltage to one or more of a gate or a source terminal of thedriving transistor during the emission operation, thereby causing thedriving transistor to convey the driving current through the lightemitting device according to the voltage stored on the storage device.

Generally, in the pixel 22, the driving current that is conveyed throughthe light emitting device by the driving transistor during the emissionoperation of the pixel 22 is a current that is supplied by the supplyline 26 i. The supply line 26 i can provide a positive supply voltage(e.g., the voltage commonly referred to in circuit design as “VDD”).

The display system 10 also includes a readout circuit 15 which isintegrated with the source driver 14. With reference again to the topleft pixel 22 in the display panel 20, the data line 23 j connects thepixel 22 to the readout circuit 15. The data line 23 j allows thereadout circuit 15 to measure a current associated with the pixel 22 andhereby extract information indicative of a degradation of the pixel 22.Readout circuit 15 converts the associated current to a correspondingvoltage. This voltage is converted into a 10 to 16 bit digital code andis sent to the digital control 16 for further processing orcompensation.

FIG. 2 is a circuit diagram of a simple individual driver circuit 50which contains a pixel 22, a source driver 14 and three switchescontrolling by MEAS 66, EM 68 and PE 64 signal. The pixel 22 in FIG. 2include a drive transistor T1 coupled to an organic light emittingdevice D1 and a storage capacitor C_(S) for storing programminginformation and allowing the pixel circuit 22 to drive the lightemitting device after being addressed. In FIG. 2, circuit 50 is inprogramming mode.

As explained above, each pixel 22 in the display panel 20 in FIG. 1 isdriven by the method shown in the driver circuit 50 in FIG. 2.The drivercircuit 50 includes a drive transistor T1 coupled to an organic lightemitting device D1, a storage capacitor C_(S) for storing programminginformation and a source driver 14 and three switches controlling byMEAS 66, EM 68 and PE 64 signal. In this example, the organic lightemitting device D1 is a luminous organic material which is activated bycurrent flow and whose brightness is a function of the magnitude of thecurrent. A supply voltage input 54 is coupled to the drain of the drivetransistor T1. The supply voltage input 54 in conjunction with the drivetransistor T1 supplies current to the light emitting device D1. Thecurrent level may be controlled via the source driver 14 in FIG. 1. Inone example, the drive transistor T1 is a thin film transistorfabricated from hydrogenated amorphous silicon. In another example,low-temperature polycrystalline-silicon thin-film transistor(“LTPS-TFT”) technology can also be used. Other circuit components suchas capacitors and transistors (not shown) may be added to the simpledriver circuit 50 to allow the pixel to operate with various enable,select and control signals such as those input by the gate driver 12 inFIG. 1. Such components are used for faster programming of the pixels,holding the programming of the pixel during different frames and otherfunctions.

When the pixel 22 is required to have a defined brightness inapplications, the gate of the drive transistor T1 is charged to avoltage where the transistor T1 generates a corresponding current toflow through the organic light emitting device (OLED) D1, creating therequired brightness. The voltage at the gate of the transistor T1 can beeither created by direct charging of the node with a voltage orself-adjusted with an external current.

During the programming mode, rows of pixels 22 are selected on a row byrow basis. For example, the “ith” row of pixels 22 are selected andenabled by the gate driver 12, in which the EM signal line 44 i is setto zero, i.e. EM=0. All pixels 22 in the “ith” row are connected to thesource driver 14, such that the MEAS signal line 42 is set to zero, i.e.MEAS=0, and the PE signal line 40 is set to equal VDD, i.e. PE=VDD, forthe “ith” row. The data is converted to data current, referred to asI_DATA 56 and flows into pixel. This data current 56 generates a Vgsvoltage in T1 transistor which is stored in C_(S) capacitor. When thepixel is in operational mode and is connected VDD, the voltage stored inC_(S) capacitor generated a current in T1 transistor which is equal toI_DATA 56.

FIG. 3 is the circuit diagram of the simple individual driver circuit 50as illustrated in FIG. 2 when in measurement mode. During themeasurement mode, each row of pixels 22 are selected on a row by rowbasis, and enabled by the gate driver 11, i.e. EM=0, and all pixels 22are connected to the source driver 14, i.e. MEAS=0 and PE=VDD, asdescribed in FIG. 2. The pixel current, I_Pixel, 70 flows into sourcedriver 14 and is measured by a Readout Circuit (ROC) 15. The ROC 15measures the pixel current 70 and converts it to a correspondencevoltage. This voltage is converted to 10 to 16 bit digital code and issent to digital processor to be used for further processing orcompensation.

FIG. 4 is the circuit diagram of the simple individual driver circuit 50as illustrated in FIG. 2 when in normal operation mode. Normal operationmode may occur after the programming of all the rows. During normaloperation mode, all pixels 22 are connected to their specific supplyline, e.g. the “ith” row is connected to supply line 26 i, while allpixels are disconnected from source driver 14, such that the MEAS signalline 42 is set to VDD, i.e. MEAS=VDD, and the PE signal line 40 is setto equal zero, i.e. PE=0, for the “ith” row. Pixel current, I_Pixel, 70which is equal to the data current, I_Data, 56 flows into pixel 22 andOLED D1 has a luminance correspondence to the Pixel current 70.

FIG. 5 is the circuit diagram of the simple individual driver circuit 50as illustrated in FIG. 2 when in programming mode but when theprogramming is directed toward another row. During the programming mode,the programming is performed on a row by row basis. The results in onlyone row of pixels 22, i.e. the “ith” row, being connected to sourcedriver 14 while the remaining rows of pixels 22, i.e. the “jth” row, areoff with no pixel current 70. During this time, the EM signal line 44 jis set to VDD, i.e. EM=VDD, while the MEAS signal line 42 is set tozero, i.e. MEAS=0, and the PE signal line 40 is set to equal VDD, i.e.PE=VDD, for the “ith” row. During this time, there will be only aleakage current flowing into the OLED D1 and pixel 22 as shown in FIG.5.

FIG. 6 is a diagram of an exemplary display system 100. The displaysystem 100 includes a gate driver 112, a source driver 114, a digitalcontroller 116, a memory storage 118, and display panel 120 and two TFTtransistors 119 working as switches for each column. The display panel120 includes an array of pixels 122 arranged in rows and columns. Eachof the pixels 122 is individually programmable to emit light withindividually programmable luminance values. The controller 116 receivesdigital data indicative of information to be displayed on the displaypanel 120. The controller 116 sends signals 132 to the source driver 114and scheduling signals 134 to the gate driver 112 to drive the pixels122 in the display panel 120 to display the information indicated. Theplurality of pixels 122 associated with the display panel 120 thuscomprise a display array (“display screen”) adapted to dynamicallydisplay information according to the input digital data received by thecontroller 116. The display screen can display, for example, videoinformation from a stream of video data received by the controller 116.The supply voltage 124 can provide a constant power voltage or can be anadjustable voltage supply that is controlled by signals from thecontroller 116.

For illustrative purposes, the display system 100 in FIG. 6 isillustrated with only four pixels 122 in the display panel 120. It isunderstood that the display system 100 can be implemented with a displayscreen that includes an array of similar pixels, such as the pixels 122,and that the display screen is not limited to a particular number ofrows and columns of pixels. For example, the display system 100 can beimplemented with a display screen with a number of rows and columns ofpixels commonly available in displays for mobile devices, monitor-baseddevices, and/or projection-devices.

The pixel 122 is operated by a driving circuit (“pixel circuit”) thatgenerally includes a driving transistor and a light emitting device.Hereinafter the pixel 122 may refer to the pixel circuit. The lightemitting device can optionally be an organic light emitting diode(OLED), but implementations of the present disclosure apply to pixelcircuits having other electroluminescence devices, includingcurrent-driven light emitting devices. The driving transistor in thepixel 122 can optionally be an n-type or p-type amorphous siliconthin-film transistor, but implementations of the present disclosure arenot limited to pixel circuits having a particular polarity of transistoror only to pixel circuits having thin-film transistors. The pixelcircuit 122 can also include a storage capacitor for storing programminginformation and allowing the pixel circuit 122 to drive the lightemitting device after being addressed. Thus, the display panel 120 canbe an active matrix display array.

As illustrated in FIG. 6, the pixel 122 illustrated as the top-leftpixel in the display panel 120 is coupled to a power enable (PE) signalline 140, measurement (MEAS) signal line 142, a supply line 126 j, adata line 123 j, and a write (WR) signal line 144 i. The supply line 126j may be charged with VDD.

The top-left pixel 122 in the display panel 120 can correspond a pixelin the display panel in an “ith” row and “jth” column of the displaypanel 120. Similarly, the top-right pixel 122 in the display panel 120represents an “ith” row and “mth” column; the bottom-left pixel 122represents an “nth” row and “jth” column; and the bottom-right pixel 122represents an “nth” row and “mth” column. Each of the pixels columns isconnected to two TFTs 119. One TFT 119 is coupled between the data line(123 j and 123 m) and pixel supply voltage line (121 j and 121 m) and iscontrolled by the PE signal line 140. The second TFT is coupled betweenpixel supply voltage line (121 j and 121 m) and supply voltage line (126j and 126 m) and is controlled by the MEAS signal line 142; The displaypanel 120 is also coupled with the appropriate supply lines (e.g., thesupply lines 126 j and 126 m), data lines (e.g., the data lines 123 jand 123 m), and write WR signal lines (e.g., the WR signal lines 144 iand 144 n). It is noted that aspects of the present disclosure apply topixels having additional connections, such as connections to a selectline or monitor line.

With reference to the top-left pixel 122 shown in the display panel 120,PE signal line 140, MEAS signal line 42 and W1R (144 i and 144 n) writesignal are provided by the gate driver 112 land can be utilized toenable, for example, a programming operation of the pixel 122 byactivating TFT transistors 119 and other switches or transistors inpixel 122 to allow the data line 123 j to program the pixel 122. Thedata line 123 j conveys programming information from the source driver114 to the pixel 122. For example, the data line 123 j can be utilizedto apply a programming voltage or a programming current to the pixel 122in order to program the pixel 122 to emit a desired amount of luminance.The programming voltage (or programming current) supplied by the sourcedriver 114 via the data line 123 j is a voltage (or current) appropriateto cause the pixel 122 to emit light with a desired amount of luminanceaccording to the digital data received by the controller 116. Theprogramming voltage (or programming current) can be applied to the pixel122 during a programming operation of the pixel 122 so as to charge astorage device within the pixel 122, such as a storage capacitor,thereby enabling the pixel 122 to emit light with the desired amount ofluminance during an emission operation following the programmingoperation. For example, the storage device in the pixel 122 can becharged during a programming operation to apply a voltage to one or moreof a gate or a source terminal of the driving transistor during theemission operation, thereby causing the driving transistor to convey thedriving current through the light emitting device according to thevoltage stored on the storage device.

Generally, in the pixel 122, the driving current that is conveyedthrough the light emitting device by the driving transistor during theemission operation of the pixel 122 is a current that is supplied by thesupply line 126 j. The supply line 126 j can provide a positive supplyvoltage (e.g., the voltage commonly referred to in circuit design as“VDD”).

The display system 100 also includes a readout circuit 115 which isintegrated with the source driver 114. With reference again to the topleft pixel 122 in the display panel 120, the data line 123 j connectsthe pixel 122 to the readout circuit 115. The data line 123 j allows thereadout circuit 115 to measure a current associated with the pixel 122and hereby extract information indicative of a degradation of the pixel122. Readout circuit 115 converts the associated current to acorresponding voltage. This voltage is converted into a 10 to 16 bitdigital code and is sent to the digital control 116 for furtherprocessing or compensation.

FIG. 7 is a circuit diagram of a simple individual driver circuit 200which contains a pixel 122 which is connected to supply voltage VDD 154,a data voltage VDATA 156 and is controlled by the write WR signal 158.The pixel 122 in FIG. 2 includes a switch transistor T2, a drivetransistor T1 coupled to an organic light emitting device (OLED) D1, theswitch transistor T2 and a storage capacitor C_(S) for storingprogramming information and allowing the pixel circuit 122 to drive thelight emitting device after being addressed. In FIG. 7, when the writeWR signal 158 goes low, it enables the transistor T2 and the VDATA 156is stored on the capacitor C_(S). The Vgs (gate to source) voltage ofthe drive transistor T1 which is stored on the capacitor C_(S) is equalto:

Vgs=VDATA-VDD

As explained above, each pixel 122 in the display panel 120 in FIG. 6 isdriven by the method shown in the driver circuit 200 in FIG. 7. Thedriver circuit 200 includes a switch transistor T2, a drive transistorT1 coupled to an organic light emitting device (OLED) D1, a storagecapacitor C_(S) for storing programming information. VDATA 156 voltagecomes from the source driver 114 and is stored on the capacitor C_(S).The switch transistor T2 is controlled by WR 58 signal. In this example,the organic light emitting device (OLED) D1 is a luminous organicmaterial which is activated by current flow and whose brightness is afunction of the magnitude of the current. A supply voltage input 154 iscoupled to the source (or drain) of the drive transistor T1. The supplyvoltage input 154 in conjunction with the drive transistor T1 suppliescurrent to the light emitting device D1. The current level may becontrolled via the source driver 114 in FIG. 6 and can be determined bythe following formula:

I _(Pixel)=½k(VDATA-VDD-V _(th))²

Where k depends on the size of the drive transistor T1 and V_(th) is thethreshold voltage of the drive transistor T1. In one example, the drivetransistor T1 is a thin film transistor fabricated from hydrogenatedamorphous silicon. In another example, low-temperaturepolycrystalline-silicon thin-film transistor (“LTPS-TFT”) technology canalso be used. Other circuit components such as capacitors andtransistors (not shown) may be added to the simple driver circuit 200 toallow the pixel to operate with various enable, select and controlsignals such as those input by the gate driver 112 in FIG. 6. Suchcomponents are used for faster programming of the pixels, holding theprogramming of the pixel during different frames and other functions.

When the pixel 122 is required to have a defined brightness inapplications, the gate of the drive transistor T1 is charged to avoltage where the transistor T1 generates a corresponding current toflow through the organic light emitting device (OLED) D1, creating therequired brightness. The voltage at the gate of the transistor T1 can beeither created by direct charging of the node with a voltage orself-adjusted with an external current.

During the programming mode, rows of pixels 122 are selected on a row byrow basis. For example, the “ith” row of pixels 122 are selected andenabled by the gate driver 112, in which the WR signal line 144 i is setto zero, i.e. WR=0. All pixels 122 in the “ith” row are connected to thesource driver 114, such that the MEAS signal line 142 is set to VDD,i.e. MEAS=VDD, and the PE signal line 140 is set to equal 0, i.e. PE=0,for the “ith” row. The data VDATA (123 j and 123 m) as a voltage (or canbe a current) is stored on the capacitors C_(S) inside pixels 122. Thisdata generates a Vgs voltage in T1 transistor which is stored in C_(S)capacitor. When the pixel is in operational mode and is connected VDD,the voltage stored in C_(S) capacitor generated a current in T1transistor which is equal to:

I _(Pixel)=½k(VDATA-VDD-V_(th))²

Pixel current, I_(Pixel), flows into pixel 122 and OLED D1 has aluminance correspondence to the Pixel current.

FIG. 8 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) 300 in programming modes. During the this mode, each rowof the circuit 300 are selected on a row by row basis and enabled by thegate driver 112 in which the WR signal line 144 i is set to zero, i.e.WR=0, and all pixels 122 are connected to the source driver 114 and thesupply voltage VDD. The MEAS signal line 142 is set to VDD, i.e.MEAS=VDD, and the PE signal line 140 is set to equal 0, i.e. PE=0, asdescribed in FIG. 8. In the first write mode 301, the write signal WR[1]is set to zero, i.e. WR[1]=0, and the row 1 is connected to the sourcedriver 114 and the data VDATA[j] 123 j is stored in capacitor C_(S) inpixel in the row 1 and the “jth” column. In the second write mode 302,the write signal WR[2] is set to zero, i.e. WR[2]=0, and the row 2 isconnected to the source driver 114 and the data VDATA[j] 123 j is storedin capacitor C_(S) in pixel in the row 2 and the “jth” column. In thethird write mode 303, the write signal WR[i] (i=3 to n-1) is set to zeroone by one, i.e. WR[i]=0 (i=3 to n-1), and the row i (i=3 to n-1) isconnected to the source driver 114 one by one and the data VDATA[j] 123j is stored in capacitor C_(S) in pixel in the “ith” row and the “jth”column. In the fourth write mode 304, the write signal WR[n] is set tozero, i.e. WR[n]=0, and the row n is connected to the source driver 114and the data VDATA[j] 123 j is stored in capacitor C_(S) in pixel in therow n and the “jth” column.

In order to measure the pixel current, in the first step, all data lineVDATA (123 j and 123 m) are set to have the same voltage as supplyvoltage (VDD) and all write signal WR (144 i and 144 n) are set to zero,i.e. WR[i]=0 (i=1 to n), then all capacitors' voltages inside pixel 122will be zero and OLED devices D1 show black color. In the second step,the leakage current is measured. In the third step, the data isprogrammed on the row i. Finally, the row i is selected and the pixelcurrent is measured.

FIG. 9 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) 400 in programming mode. In first step, data line VDATA123 j has the same voltage as supply voltage VDD 126 j. All writesignals WR (144 i, 144 n) are set to zero, i.e. WR=0, and the MEASsignal line 142 is set to VDD, i.e. MEAS=VDD, and the PE signal line 140is set to equal 0, i.e. PE=0, as described in FIG. 9. All pixels 122 inthe circuit 400 are in write mode 401. All capacitors' voltages are setto zero and OLED devices D1 show black color. Alternatively all of thepixels can be driven to black one at a time sequentially similar to howthe video is driven onto the panel.

FIG. 10 is a block diagram of an embodiment of a column of pixel circuit(“jth” column) 500 in measurement mode. In the second step, the leakagecurrent is measured immediately after setting the capacitors' voltagesof all pixels in the circuit 500 to zero. The WR signal line (144 i and144 n) is set to VDD, i.e. WR=VDD, and the MEAS signal line 142 is setto 0, i.e. MEAS=0, and the PE signal line 140 is set to equal VDD, i.e.PE=VDD, as described in FIG. 10. The circuit 500 is disconnected fromthe supply voltage and connected to the data line, VDATA 123 j. Theleakage current of the pixels 122 in “jth” column (the circuit 500),I_(Leakage) 190 flows into the source driver 114 and is measured by aReadout Circuit (ROC) 115. The ROC 115 measures the leakage current(I_(Leakage)) 190 and converts it to a correspondence voltage. Thisvoltage is converted to 10 to 16 bit digital code and is sent to digitalprocessor to be used for further processing or compensation.

The third step is to write a data into the pixel which is of interestedto measure its current. FIG. 11 is a block diagram of an embodiment of acolumn of pixel circuit (“jth” column) 600 in programming mode. In thismode the “ith” row is programmed. The WR signal line 144 i is set tozero, i.e. WR[i]=0, and other WR signal lines 144 n are set to equalVDD, i.e. WR[n]=VDD, and the MEAS signal line 142 is set to equal VDD,i.e. MEAS=VDD, and the PE signal line 140 is set to zero, i.e. PE=0, asdescribed in FIG. 11. The pixel 122 in “ith” row is programmed to VDATA123 j and a current corresponded to it flows into the pixel. No currentexcept for the leakage current flow into other pixel 122 in “jth”column.

The last step is to measure the pixel current of the “ith” row. FIG. 12is a block diagram of an embodiment of a column of pixel circuit (“jth”column) 700 in measurement mode. The pixel current of the “ith” row plusthe leakage current of the other pixels are measured in this mode. TheWR signal line (144 i and 144 n) is set to VDD, i.e. WR=VDD, and theMEAS signal line 142 is set to 0, i.e. MEAS=0, and the PE signal line140 is set to equal VDD, i.e. PE=VDD, as described in FIG. 12. Thecircuit 700 is disconnected from the supply voltage and connected to thedata line, VDATA 123 j. The pixel current of the “ith” row plus theleakage current of other pixels in “jth” column (the circuit 700),I_(Pixel)+I_(Leakage), 192 flows into the source driver 114 and ismeasured by a ROC 115. The ROC 115 measures the current 192 and convertsit to a correspondence voltage. This voltage is converted to 10 to 16bit digital code. The difference between the current measured in thelast step and the leakage current in the step two, is the pixel currentof the “ith” row pixel in “jth” column circuit 700 according to thefollowing formula:

I _(Pixel)=(current measured in step 4)-(current measured in step 2)

I _(Pixel)=(I _(Pixel) +I _(Leakage))-(I _(Leakage))

In order to measure the OLED current, all four steps described tomeasure the pixel current are repeated here. In the step one as shown inFIG. 9, the data line is set to equal VDD and the capacitors' voltagesinside pixels are set to zero. In the step two as shown in FIG. 10, theleakage current, I_(Leakage), 190 of the pixels is measured. In the stepthree as shown in FIG. 11, the “ith” row is selected and the data lineVDATA 123 j is derived with lowest voltage. It causes the T1 transistorinside the “ith” pixel 122 is pushed to the triode region and behaveslike a switch. In the step four as shown in FIG. 8, the OLED D1 of the“ith” pixel 122 is connected to virtual ground 806 of an integrator 810through the T1 transistor inside the “ith” pixel 122 and the transistor119 connected between the pixel supply voltage node 121 j and the dataline 123 j and the switch 807 inside the ROC 115. By ignoring thevoltage drop on the switches, the OLED D1 of the “ith” pixel 122 willhave the same voltage as the bias voltage V_(B) 805. The OLED current ofthe “ith” row pixel plus the leakage current of other pixels in “jth”column (the circuit 800), I_(Oled)+I_(Leakage), 194 flows into thesource driver 114 and is measured by a ROC 115. The ROC 115 measures thecurrent 194 and converts it to a correspondence voltage. This voltage isconverted to 10 to 16 bit digital code 802. The difference between thecurrent measured in the step four and the leakage current in the steptwo, is the OLED current of the “ith” row pixel in “jth” column circuit800 according to the following formula:

I _(Oled)=(current measured in step 4)-(current measured in step 2)

I _(Oled)=(I _(Oled) +I _(Leakage))-(I _(Leakage))

The ROC 115 as shown in FIG. 13 includes one switch 807, an integrator810 and an analog to digital converter (ADC) 801. The integratorincludes a reset switch 808, an integrating capacitor C^(I) and a biasvoltage V_(B) 805. The integrator integrates the current coming frompixel 122 and converts it to a corresponding voltage. The voltage isconverted to 10 to 16 bit digital code 802 by the ADC 801.

While particular embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise construction and compositionsdisclosed herein and that various modifications, changes, and variationscan be apparent from the foregoing descriptions without departing fromthe spirit and scope of the invention as defined in the appended claims.

1. A method of determining a current flowing in a display system, thedisplay system including a plurality of pixel circuits arranged in rowsand columns, a first pixel circuit of the plurality of pixel circuitscoupled to a data line and coupled to a pixel supply voltage line, afirst transistor coupled to the first pixel circuit via the data lineand via the pixel supply voltage line, the method comprising: during atleast one mode of operation of the first pixel circuit, turning on thefirst transistor to couple the data line to the pixel supply voltageline; and measuring over the data line a current flowing over the pixelsupply voltage line through the first transistor during the at least onemode of operation.
 2. The method of claim 1, wherein the data line iscoupled to more than one pixel circuit of the plurality of pixelcircuits, and said first transistor is coupled to said more than onepixel circuit via the data line.
 3. The method of claim 2, wherein thepixel supply voltage line is coupled to said more than one pixel circuitof the plurality of pixel circuits, and said first transistor is coupledto said more than one pixel circuit via the pixel supply voltage line.4. The method of claim 1, wherein the display system includes a voltagesupply for providing a supply voltage, and a supply voltage transistorcoupled between the supply voltage and the pixel supply voltage line,the method further comprising: during the at least one mode of operationof the first pixel circuit, turning off the supply voltage transistor todecouple the supply voltage from the pixel supply voltage line.
 5. Themethod of claim 4, further comprising: during a first at least one modeof operation of the first pixel circuit, measuring over the data line aleakage current flowing over the pixel supply voltage line and throughthe first transistor.
 6. The method of claim 5, further comprising:prior to a second at least one mode of operation of the first pixelcircuit, programming the first pixel circuit over the data line; andduring the second at least one mode of operation, measuring over thedata line a combination of the leakage current and a current flowingthrough the first pixel circuit over the pixel supply voltage line andthrough the first transistor.
 7. The method of claim 6, furthercomprising: determining the current flowing through the first pixelcircuit with use of a difference between the measured leakage currentand the measured combination of the leakage current and the currentflowing through the first pixel circuit.
 8. The method of claim 7,wherein each pixel circuit comprises an organic light-emitting diode(OLED), and a drive transistor, the method further comprising: duringthe second at least one mode of operation supplying the current flowingthrough the first pixel circuit to the OLED with use of the drivetransistor according to said programming of the first pixel circuit. 9.The method of claim 8, wherein the first pixel circuit is programmedsuch that during the second at least one mode of operation the drivetransistor operates in the triode region, and the current flowingthrough the first pixel circuit corresponds to the OLED current.
 10. Themethod of claim 9, wherein the display system comprises a readoutcircuit, and wherein the readout circuit performs said measuring.
 11. Adisplay system comprising: a plurality of pixel circuits arranged inrows and columns; a data line; a pixel voltage supply line; a firstpixel circuit of the plurality of pixel circuits coupled to the dataline and coupled to the pixel supply voltage line; a first transistorcoupled to the first pixel circuit via the data line and via the pixelsupply voltage line; and a controller adapted to control the pluralityof pixels and the first switch, the controller further adapted to:during at least one mode of operation of the first pixel circuit, turnon the first transistor to couple the data line to the pixel supplyvoltage line; and measure over the data line a current flowing over thepixel supply voltage line through the first transistor during the atleast one mode of operation.
 12. The display system of claim 11, whereinthe data line is coupled to more than one pixel circuit of the pluralityof pixel circuits, and said first transistor is coupled to said morethan one pixel circuit via the data line.
 13. The display system ofclaim 12, wherein the pixel supply voltage line is coupled to said morethan one pixel circuit of the plurality of pixel circuits, and saidfirst transistor is coupled to said more than one pixel circuit via thepixel supply voltage line.
 14. The display system of claim 11, furthercomprising: a voltage supply for providing a supply voltage; and asupply voltage transistor coupled between the supply voltage and thepixel supply voltage line, wherein the controller is further adapted to:during the at least one mode of operation of the first pixel circuit,turn off the supply voltage transistor to decouple the supply voltagefrom the pixel supply voltage line.
 15. The display system of claim 14,wherein the controller is further adapted to: during a first at leastone mode of operation of the first pixel circuit, measure over the dataline a leakage current flowing over the pixel supply voltage line andthrough the first transistor.
 16. The display system of claim 15,wherein the controller is further adapted to: prior to a second at leastone mode of operation of the first pixel circuit, program the firstpixel circuit over the data line; and during the second at least onemode of operation, measure over the data line a combination of theleakage current and a current flowing through the first pixel circuitover the pixel supply voltage line and through the first transistor. 17.The display system of claim 16, wherein the controller is furtheradapted to: determine the current flowing through the first pixelcircuit with use of a difference between the measured leakage currentand the measured combination of the leakage current and the currentflowing through the first pixel circuit.
 18. The display system of claim17, wherein each pixel circuit comprises an organic light-emitting diode(OLED), and a drive transistor, and wherein during the second at leastone mode of operation, the current flowing through the first pixelcircuit is supplied to the OLED by the drive transistor according tosaid programming of the first pixel circuit.
 19. The display system ofclaim 18, wherein the controller programs the first pixel circuit suchthat during the second at least one mode of operation the drivetransistor operates in the triode region, and the current flowingthrough the first pixel circuit corresponds to the OLED current.
 20. Thedisplay system of claim 19 further comprising a readout circuit, andwherein the controller controls said readout circuit to perform saidmeasuring.